After completion of the drilling of an oil or gas well, the well casing, set during the drilling operation, must be perforated so that the oil and/or gas can enter the production tubing and be produced. Various methods are presently used for perforating the well casing. Generally, some type of jet or bullet gun device is lowered into the well casing, which, upon detonation, perforates the casing and permits the oil and/or gas to flow from the reservoir into the well.
To perforate a well casing, it is necessary to position the perforator device adjacent the oil and/or gas-bearing stratum. With the prior perforating method of using a high differential pressure in the well bore and a relatively large casing gun, the perforator is attached to the end of production tubing and inserted into the well casing. The length of the production tubing is adjusted to position the perforator device opposite the preselected stratum after a second radioactive correlation log is run inside the tubing. This is a time-consuming and expensive process because it often requires many short "subs" of pipe to adjust the production tubing so that the perforator will be positioned properly.
One prior method for perforating a well with a casing gun and differential pressure, as disclosed in U.S. Pat. No. 4,113,016, is to deposit the perforator in the well prior to running the production tubing. After insertion of the production tubing, a collar locator and connector tools are lowered into the well on a wireline and connected to the perforator. The collar locator is used to position the perforator adjacent the selected stratum. This prior method requires a special trip into the hole to deposit the perforator.
In high differential-to-the-well bore pressure perforation, a large pressure differential exists from the oil and/or gas-bearing stratum to the well casing. When using a perforator on a wireline cable in the performation of a well, the inrushing flow of oil and/or gas attempts to force the perforating device up the well casing, with the result that the device can become jammed or fouled with the wireline cable in the well tubing or casing. An expensive and time-consuming fishing operation may then be required to remove the fouled cable and perforator. Thus, perforation under high pressure differential-to-the-well bore conditions using a perforator on a wireline has not been reliably performed with prior perforating devices.
High pressure differential-to-the-well bore perforating, using a casing gun, despite difficulties in operation, is highly desirable. A well perforated at a high pressure differential-to-the-well bore is more productive than one perforated at a low pressure differential and a well perforated with a casing gun is more productive than a well perforated with a smaller gun. Two reasons are presently advanced to explain the higher productivity. First, when the perforator detonates, each jet-cut hole is thoroughly cleansed, that is, the "carrot" (a piece of copper lining from the jet left in the hole) with the crushed and melted debris resulting from the detonation are cleared from each opening and all perforations are cleared out to produce. Second, the casing gun makes larger perforations and deeper holes. With a high pressure differential, debris is immediately driven from the perforated hole into the casing to be produced, and not into the oil and/or gas-bearing strata. This prevents contamination of the oil and/or gas-bearing stratum, thereby resulting in a more productive oil and/or gas well.